1. Field of the Invention
The present invention relates in general to a method of and a system for detecting a location within an enclosed vessel at which a sound signal is produced. The method according to the invention is particularly suited for locating an inner portion of a pressure vessel of a nuclear reactor at which a sound is produced due to collision of a loose part against that portion.
2. Description of the Prior Art
Collision of a so-called loose part such as an accidentally introduced metal part or a part detached from an inner structure with inner parts of a pressure vessel of a nuclear reactor is not only a transient phenomenon the occurrence of which can not be predicted but also a transitory phenomenon in the sense that it may not take place again once it has occurred. It is important for the safety operation of a nuclear plant to catch the transient and transitory phenomenon and detect with a high reliability when it takes place due to the collision of loose parts. In the case of a nuclear reactor in operation, there are present background noises such as electric noises and notch noise produced upon driving the control rods. Thus the sound produced as a result of collision of a loose part with the vessel has to be discriminated from these background noises. In this connection, it is noted that the notch noise produced upon driving the control rod, for example, inherently has a high similarity to the sound produced by a loose part, making it difficult to discriminate the latter from the former.
Heretofore, as an approach for detecting the sound produced by a loose part, a filter and a peak value discriminator have been used, wherein the appearance of a signal level higher than a preset value is evaluated as an indication of the sound produced by the loose part. Of course, it is impossible with this approach to discriminate the sound due to collision of the loose part from the background noises described above. As another attempt, it has been proposed to estimate the location of a collision on the basis of the difference in time in the appearance of output signals from the sound detectors, wherein a signal having a particular time difference is utilized as the signal indicating the abnormal sound. This method is certainly effective for the objects such as conduits and a container of a simple structure which can be dealt with one- or two-dimensionally. However, it is impossible to estimate with a reasonable accuracy the location at which the sound due to collision is produced in a complicated container such as a pressure vessel of a nuclear reactor because of an increased number of much complicated sound propagation paths.